1. Field of the Invention
This invention relates to a moving body measuring apparatus for measuring a distance, an azimuth or the like to a moving body such as a vehicle using a laser beam.
2. Description of the Related Art
An obstacle detection system for vehicles which is carried on a vehicle to detect an obstacle using a laser beam is already known and disclosed, for example, in U.S. Pat. No. 4,477,184. The obstacle detection system two-dimensionally scans spot light of a pulse modulated laser beam by means of acousto-optic light deflectors so as to irradiate upon a stationary obstacle ahead of the vehicle or a target such as another vehicle running ahead of the running vehicle. Reflected light from the target is received and photo-electrically converted by a photo-detector, and an azimuth and a distance to the target at the laser beam reflecting point are calculated in accordance with an output of the photo-detector by arithmetic logic means. Then, the arithmetic logic means decides in accordance with a result of the calculation and a vehicle velocity signal from the vehicle whether a possibility of collision exists or whether the passing-through is possible.
The obstacle detection system for vehicles is advantageous in that, because a laser beam is contracted to a small directional angle, the power intensity of a laser beam coming upon a target is so high that a measurement of an object at a long distance can be achieved, and not only a distance and an azimuth to the target but also three-dimensional information such as a size (particularly in a vertical direction) of the target can be obtained. However, the obstacle detection system has such disadvantages as described below.
First, since a spot-like laser beam is two-dimensionally scanned in both horizontal and vertical directions to obtain measurement data for a screen, the number of emissions of a laser beam and the number of measurements per one frame are very great, and the time required for measurement per one frame is very long. In order to reduce the time, the period of laser oscillation must necessarily be reduced. However, the reduction in period is limited due to construction of laser oscillation, and consequently, a significant long period of time is required for measurement per one frame. Accordingly, if the distance or azimuth from the vehicle to the target changes during measuring processing for one screen, then part of data will become outdated. As a result, accurate measurement cannot be achieved for the entire one screen, or data processing is complicated because, for example, the outdated data must be corrected. The degree increases, when the target is a running vehicle, as the difference in speed between the vehicles increases.
Second, since two-dimensional scanning is involved, a mechanism for scanning operation is complicated and large in size. In particular, since a laser beam itself is very thin, a light deflector for scanning the laser beam in one direction may be of the small size. However, another light deflector for scanning the laser beam deflected in one direction further in another perpendicular direction must necessarily be disposed in a sufficiently spaced relationship from the first light deflector in order to prevent otherwise possible interference between the light deflectors. Besides, also a mechanism for synchronizing the two light deflectors with each other is necessitated. Consequently, the entire scanning mechanism is complicated and large in size.
Third, while an acoustic-optic light deflector for diffracting a laser beam with ultrasonic waves is employed in order to assure high speed two-dimensional scanning and the deflection angle of light is changed by changing the frequency of the ultrasonic waves, it is disadvantageous in that the deflection angle is limited, that a high ultrasonic wave output is required in order to obtain a sufficiently high deflection efficiency, that it is difficult to manufacture, the power consumption is high, and that it must be cooled because light is disturbed otherwise by thermal distortion thereof by a temperature gradient caused by heat energy into which ultrasonic wave energy is converted.
Fourth, a light receiving optical system must necessarily have a field of view of a sufficient area in order to receive reflected light which is two-dimensionally scanned as described above. Consequently, a light receiving optical element has a large area, and a photo-detector likely suffers from noises as much. Further, since the light receiving field of view is great, an influence of the background light or sunlight is so high that measurement may be impossible in some cases.
In this manner, the conventional obstacle detection system for vehicles which involves two-dimensional scanning has various disadvantages in practical use and is complicated in structure and expensive. Further, when the first to fourth problems described above are taken into consideration, particularly in case the target is a running automobile, it is a question whether a satisfactory result of measurement can actually be obtained because the running automobile presents a great variation in velocity and in advancing direction.
By the way, in case the moving target is assumed to be an automobile, it will not present a significantly great positional variation in a vertical direction, and accordingly, the position thereof in a vertical direction need not necessarily be measured for practical use (for example, when the measurement is conducted principally in order to prevent a rear-end collision or to maintain a predetermined distance from another vehicle). Thus, if, when measurement data for a screen are to be obtained by scanning of a laser beam, the laser beam is expanded in a vertical direction and is scanned only in a horizontal direction, then although data of a position in a vertical direction cannot be obtained, the time required for scanning and measurement for one screen can be decreased significantly and besides the mechanism for such scanning and measurement is simplified considerably comparing with the conventional measurement which involves two-dimensional scanning in vertical and horizontal directions.